A traditional technique for recovering precious metal(s) from precious metal-containing ore is by leaching the material with a cyanide lixiviant. As used herein, a “precious metal” refers to gold, silver, and the platinum group metals (e.g., platinum, palladium, ruthenium, rhodium, osmium, and iridium). Many countries are placing severe limitations on the use of cyanide due to the deleterious effects of cyanide on the environment. Incidents of fish and other wildlife having been killed by the leakage of cyanide into waterways have been reported. The limitations being placed on cyanide use have increased substantially the cost of extracting precious metal(s) from ore, thereby decreasing precious metal reserves in many countries. Cyanide is also unable to recover precious metals such as gold from refractory ores without a pretreatment step. “Refractory ores” refer to those ores that do not respond well to conventional cyanide leaching. Examples of refractory ores include sulfidic ores (where at least some of the precious metals are locked up in the sulfide matrix), carbonaceous ores (where the precious metal complex dissolved in the lixiviant adsorbs onto carbonaceous matter in the ores), and cres that are both sulfidic and carbonaceous.
Thiosulfate has been actively considered as a replacement for cyanide. Thiosulfate is relatively inexpensive and is far less harmful to the environment than cyanide. Thiosulfate has also been shown to be effective in recovering precious metals from pretreated refractory preg-robbing carbonaceous ores and sulfidic ores. As used herein, “preg-robbing” is any material that interacts with (e.g., adsorbs or binds) precious metals after dissolution by a lixiviant, thereby interfering with precious metal extraction, and “carbonaceous material” is any material that includes one or more carbon-containing compounds, such as, but not limited to, humic acid, graphite, bitumins and asphaltic compounds.
Where gold is the precious metal, thiosulfate leaching techniques have typically relied on the use of copper ions to catalyze and accelerate the oxidation of gold, ammonia to facilitate the formation and stabilization of cupric ammine ions and/or a pH at pH 9 or above to maintain a region of stability where both the cupric ammine and gold thiosulfate complexes are stable.
It is well known in the art that the catalytic effect of copper and ammonia in conventional thiosulfate leaching of gold is described by the following sequence of reactions.
Formation of the cupric ammine complex:Cu2++4NH3→Cu(NH3)42 +  (1)
Oxidation of gold by cupric ammine, gold complexation as the gold-thiosulfate anion, and reduction of cupric ammine to cuprous thiosulfate:Au+Cu(NH3)42++5S2O32−→Au(S2O3)23−+Cu(S2O3)35−4NH3  (2)
Oxidation of the cuprous thiosulfate back to cupric ammine with oxygen:Cu(S2O3)35−4NH3 +¼O2+½H2O→Cu(NH3)42++3S2O32−+OH−  (3)
Summing equations (2) and (3) yields the overall thiosulfate leach reaction for gold:Au+2S2O3−+¼O2+H2O→Au(S2O3)23−+OH−  (4)
It can be seen from the above equations that copper and ammonia act as catalysts in that they are neither produced nor consumed in the overall leach reaction.
Copper and ammonia can be a source of problems. Rapid oxidation of thiosulfate by cupric ammine to form polythionates occurs, leading to excessive degradation and loss of thiosulfate:2Cu(NH3)42++8S2O32−→2Cu(S2O3)35−+S4O62−+8NH3  (5)Oxidative degradation of thiosulfates by molecular oxygen to polythionates and sulfates is accelerated markedly in the presence of copper ions and/or ammonia. Molecular oxygen conversion to thiosulfates is believed to occur according to sequence of reactions that involve the formation of intermediate polythionates (polythionates can be represented by SnO62−, where n=2-6):Tetrathionate formation: 2S2O32−+½O2+H2O→S4O62−+2OH−  (6)Trithionate formation: 3S4O62−+ 5/2O2+H2O→S3O62−+2H+  (7)Sulfite formation: S3O62−+½O2 +2H2O→3SO32−+4H+  (8)Sulfate formation: 2SO32−+O2→2SO42−  (9)Overall: S2O32−+2O2+H2O→2SO42−+2H+  (10)Not only can the degradation of thiosulfate lead to increased reagent costs but also it has been discovered that excessive levels of sulfate can cause decreased gold recoveries. While not wishing to be bound by any theory, it is believed that excessive levels of sulfates can lead to unacceptable rates of thiosulfate degradation and levels of instability in the thiosulfate lixiviant. Additionally, ammonia gas can be released into the atmosphere when atmospheric leaching is performed. The loss of ammonia by volatilization occurs readily, particularly in unsealed gas-sparged reactors and heaps operating at pH greater than 9.2, leading to excessive ammonia consumption:NH4++OH−→NH3(aq)+H2O→NH3(g)+H2O  (11)